Nowadays, as deep-sea oil wells and gas wells are developed, there is a strong demand for high-strength heavy wall steel pipes for offshore pipelines and oil country tubular goods. Furthermore, steel pipes to be used in cold areas such as the North Sea and Alaska are strongly required to have satisfactory low temperature reliability of a weld zone of an electric resistance welded steel pipe, in particular, brittle fracture initiation resistance at a low temperature.
Usually, in a weld zone (electric resistance weld zone) of an electric resistance welded steel pipe formed as a result of performing electric resistance welding, strength (hardness) is higher and toughness is lower than in a base metal zone due to rapid heating and rapid quenching when welding is performed. In order to solve such problems in an electric resistance weld zone, recently, techniques for recovering (increasing) toughness in an electric resistance weld zone (weld zone) to the level of that in a base metal as a result of improving microstructure in an electric resistance weld zone by performing in-line heating and cooling after welding (electric resistance welding) has been performed on the electric resistance weld zone have been proposed.
For example, Patent Literature 1 describes a method for manufacturing an electric resistance welded steel pipe excellent in terms of low temperature toughness, the method including heating the electric resistance weld zone of an electric resistance welded steel pipe having a chemical composition containing C: 0.05% to 0.20%, Si: 0.3% or less, and Mn: 0.50% to 2.00% as basic constituent chemical elements and further containing one, two, or all of Nb: 0.01% to 0.10%, V: 0.01% to 0.10%, Ti: 0.01% to 0.05%, and the balance being Fe and inevitable impurities at a temperature of 850° C. to 1000° C., starting rapid quenching at a temperature equal to or higher than the Ar3 transformation point at a cooling rate of more than 30° C./s and 100° C./s or less, ending cooling at a temperature of (Ar1−50° C.) to (Ar1−100° C.), and then performing weak cooling. In Patent Literature 1, it is said that it is possible to achieve excellent toughness in an electric resistance weld zone substantially equivalent to that in a base metal without performing reheating (tempering) after cooling has been performed.
In addition, Patent Literature 2 describes a method for manufacturing a high-toughness electric resistance welded steel pipe, the method including heating the weld zone (electric resistance weld zone) of an electric resistance welded steel pipe having a chemical composition containing C: 0.10% or less, Si: 0.5% or less, Mn: 0.4% to 1.6%, P: 0.025% or less, S: 0.010% or less, Nb: 0.01% to 0.08%, Ti: 0.01% to 0.07%, V: 0.005% to 0.07%, and the balance being Fe and inevitable impurities at a temperature of 850° C. to 1050° C., cooling the heated weld zone at a cooling rate of 5° C./s to 20° C./s, and, optionally, further performing tempering by heating the cooled weld zone at a temperature of 550° C. or lower and by cooling the heated weld zone. It is said that, with this method, it is possible to achieve both high strength and high toughness in an electric resistance weld zone substantially equivalent to those in a base metal.
In addition, Patent Literature 3 describes a method for performing a heat treatment on a heavy wall electric resistance welded steel pipe. The technique described in Patent Literature 3 is a method for performing a heat treatment on a heavy wall electric resistance welded steel pipe, the method including, when a continuous heat treatment is performed on the weld zone of a heavy wall electric resistance welded steel pipe by using a high-frequency induction heating device, first performing first heating on the weld zone such that the inner surface of the weld zone has a temperature of (the Ar3 point+50° C.) or higher, then cooling the heated weld zone with water or air such that the outer surface of the weld zone has a temperature equal to or lower than the end temperature of bainite transformation of the heated material, and then performing second heating at a temperature with which it is possible for the Ac3 transformation zone to entirely cover the generating area of bainite transformation due to the first heating and cooling and which is equal to or lower than a temperature at which a bainite microstructure is generated. It is said that, with this method, it is possible to manufacture heavy wall electric resistance welded steel pipe having a weld zone quite excellent in terms of workability, toughness, and corrosion resistance, which are often used, without adding a process which is complex and requires a long treatment time.